factor out a read_object_entry

this way we can read individual object entries without reading
the entire list.

ebpfcat/ethercat.py

@@ -65,6 +65,7 @@ class ECDataType(Enum):
        obj._value_ = value
        obj.fmt = fmt
        return obj
+   INVALID = 0, None
    BOOLEAN = 0x1, "?"
    INTEGER8 = 0x2, "b"
    INTEGER16 = 0x3, "h"
@@ -166,11 +167,12 @@ class ObjectDescription:
 class ObjectEntry:
     name = None
 
-    def __init__(self, desc):
-        self.desc = desc
+    def __init__(self, terminal, index):
+        self.terminal = terminal
+        self.index = index
 
     async def read(self):
-        ret = await self.desc.terminal.sdo_read(self.desc.index, self.valueInfo)
+        ret = await self.terminal.sdo_read(self.index, self.valueInfo)
         if self.dataType in (ECDataType.VISIBLE_STRING,
                              ECDataType.UNICODE_STRING):
             return ret.decode("utf8")
@@ -188,8 +190,7 @@ class ObjectEntry:
         else:
             d = pack("<" + self.dataType.fmt, data)
 
-        return await self.desc.terminal.sdo_write(d, self.desc.index,
-                                                  self.valueInfo)
+        return await self.terminal.sdo_write(d, self.index, self.valueInfo)
 
     def __repr__(self):
         if self.name is None:
@@ -871,6 +872,21 @@ class Terminal:
                             raise EtherCatError(f"requested index {index}")
                     toggle ^= 0x10
 
+    async def read_object_entry(self, index, subidx):
+        """read a object entry from the CoE self description"""
+        data = await self.coe_request(CoECmd.SDOINFO, ODCmd.OE_REQ,
+                                      "HBB", index, subidx, 7)
+        oe = ObjectEntry(self, index)
+        oe.valueInfo, dataType, oe.bitLength, oe.objectAccess = \
+            unpack_from("<BxHHH", data)
+        assert subidx == oe.valueInfo
+        if dataType < 2048:
+            oe.dataType = ECDataType(dataType)
+        else:
+            oe.dataType = dataType
+        oe.name = data[8:].decode("utf8")
+        return oe
+
     async def read_ODlist(self):
         idxes = await self.coe_request(CoECmd.SDOINFO, ODCmd.LIST_REQ, "H", 1)
         idxes = unpack("<" + "H" * int(len(idxes) // 2), idxes)
@@ -893,22 +909,12 @@ class Terminal:
             od.entries = {}
             for i in range(1 if od.maxSub > 0 else 0, od.maxSub + 1):
                 try:
-                    data = await self.coe_request(CoECmd.SDOINFO, ODCmd.OE_REQ,
-                                                  "HBB", od.index, i, 7)
+                    oe = await self.read_object_entry(od.index, i)
                 except EtherCatError as e:
                     logging.info(f"problems reading SDO {od.index:x}:{i:x}:")
                     continue
-                oe = ObjectEntry(od)
-                oe.valueInfo, dataType, oe.bitLength, oe.objectAccess = \
-                        unpack("<BxHHH", data[:8])
-                if dataType == 0:
+                if oe.dataType is ECDataType.INVALID:
                     continue
-                assert i == oe.valueInfo
-                if dataType < 2048:
-                    oe.dataType = ECDataType(dataType)
-                else:
-                    oe.dataType = dataType
-                oe.name = data[8:].decode("utf8")
                 od.entries[i] = oe
         return ret